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Old 10-28-2008, 12:41 AM   #1
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Tripping Point?


Where is a 15A breaker suppose to trip, exactly at the moment it hits 15A of use, what about motor startup surge?

I had a clamp meter on a wire tonight on my new porch wiring, I had the lighting on (inside and out) and the fridge is temporally plugged in on the porch. It is a 14G, 15A circuit. The clamp meter read about 15.5 to 15.75 when the fridge was on and my shop vac was running, with the lighting on. My shopvac is the heaviest part of the load, the regular load, of all the lights is about 5 to 6 Amps. My real question is, should the breaker have tripped, if not, when should it?

I believe my clamp meter is accurate, as I have compared it to my kill a watt and to a 100W light bulb and they all get very similar results (+/-1-2% in my estimate).
What I am kind of worried about is what if I had this very same setup once I upgrade my old pushmatic panel to a modern panel, like the new CH panel. Am I more likely to see it trip with this load on it then I am with the pushmatic?

Jamie

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Old 10-28-2008, 05:44 AM   #2
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Tripping Point?


A good rule of thumb is don't exceed 80% of your breaker for a " continuous " load . Breakers of today are thermo-magnetic . They are designed to hold 100% for a short time . An older style ( like FPE ) may be thermal only and have to heat up before it trips .

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Old 10-28-2008, 07:18 AM   #3
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Tripping Point?


Your never supposed to use more than 80% of your circuit's amperage continuously. I'd expect an older style breaker to not trip at 15 amps exactly, some of the newer breakers probably should trip earlier.
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Old 10-28-2008, 09:21 AM   #4
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Tripping Point?


Quote:
Originally Posted by rgsgww View Post
Your never supposed to use more than 80% of your circuit's amperage continuously. I'd expect an older style breaker to not trip at 15 amps exactly, some of the newer breakers probably should trip earlier.
So is "continously" for things that are fixed like lights and installed appliances?

I mean a 10A draw from a shop vac puts a 15A circuit over 80% if only 3 amps of lights are running.

Jamie
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Old 10-28-2008, 10:51 AM   #5
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Tripping Point?


Circuit breakers are designed for short circuit protection not overload protection. They will trip on overload after the breaker has time to reach the temperature limit. You have to look at amps as temperature or heat. For this very reason most all appliance motors will have their own overload protection. I am certain that if the overload condition was for an extended period of time it would have eventually tripped.
All breakers are not created equal. Your new panel may or may not respond in the same manner, but rest assured if the wiring is the correct size for the breaker, you should not be concerned.

PS.....Not all clamp meters are created equal either. Test against another clamp or have it checked and calibrated if required.
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Old 10-28-2008, 10:51 AM   #6
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Tripping Point?


Quote:
Originally Posted by jamiedolan View Post
So is "continously" for things that are fixed like lights and installed appliances?

I mean a 10A draw from a shop vac puts a 15A circuit over 80% if only 3 amps of lights are running.

Jamie
Continously means 3 hours or more.
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Old 10-28-2008, 11:35 AM   #7
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Tripping Point?


Quote:
Originally Posted by jamiedolan View Post
Where is a 15A breaker suppose to trip, exactly at the moment it hits 15A of use, what about motor startup surge?

I had a clamp meter on a wire tonight on my new porch wiring, I had the lighting on (inside and out) and the fridge is temporally plugged in on the porch. It is a 14G, 15A circuit. The clamp meter read about 15.5 to 15.75 when the fridge was on and my shop vac was running, with the lighting on. My shopvac is the heaviest part of the load, the regular load, of all the lights is about 5 to 6 Amps. My real question is, should the breaker have tripped, if not, when should it?

I believe my clamp meter is accurate, as I have compared it to my kill a watt and to a 100W light bulb and they all get very similar results (+/-1-2% in my estimate).
What I am kind of worried about is what if I had this very same setup once I upgrade my old pushmatic panel to a modern panel, like the new CH panel. Am I more likely to see it trip with this load on it then I am with the pushmatic?

Jamie
Most breakers have a trip that follows what is called the inverse-time curve. What this means is that the greater the overload, the quicker the tripping action. And the ambient temperature is important. If a breaker is supposed to trip at 15 amps at 40 degrees Celsius, then it may take 25 amps to trip it at 20 degrees C. But a slight overload isn't going to get the wire hot enough to burst into flames or melt the insulation. The 15 amp rating for #14 AWG is a conservative number.
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Old 10-28-2008, 03:28 PM   #8
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Tripping Point?


Quote:
Originally Posted by electro View Post
A good rule of thumb is don't exceed 80% of your breaker for a " continuous " load . Breakers of today are thermo-magnetic . They are designed to hold 100% for a short time . An older style ( like FPE ) may be thermal only and have to heat up before it trips .
Code states that no load shall exceed 80% of the rated capacity UNLESS it is rated for 100% use. However the UL does not have any breakers rated for 100% capacity for residential use. I will look up the code tomorrow in tech.

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Continously means 3 hours or more.
That is only part of it. It also includes something about amperage. Following your definition if I leave my lights on for 3 hours it is a continious load.
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Old 10-28-2008, 04:15 PM   #9
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Tripping Point?


From the 2005 NEC:
Continuous load. A load where the maximum current is expected to continue for 3 hours or more.

What would you expect to run for 3 hours or more in a house?
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Old 10-28-2008, 04:56 PM   #10
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Tripping Point?


Thanks for the clarification on how this works and the associated code. Ofcourse my wires / breakers are sized correctly. I decided to go ahead and run some 12/2 to a series of new outlets and drop it onto a new 20A breaker to make sure I have enough capicity at these outlets.
Thanks
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Old 10-28-2008, 05:45 PM   #11
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Tripping Point?


http://www.accontrols.com/Typical%20...ip%20Curve.pdf
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Old 10-28-2008, 05:45 PM   #12
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You can load a breaker 100%, and i doubt there is anything in residential that would qualify for a continuous load.
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Old 10-28-2008, 11:49 PM   #13
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Tripping Point?


Looking at the spec book for a Square D QO115 (single pole 15 amp), the instantaneous (magnetic) part will trip the breaker if the current is between 7X and 15X (105 and 225 amps) or higher, for longer than 1 cycle (1/60th of a second).

The thermal part will trip as follows;
5X to 7X for 1 second
3.5X to 6X for 2 seconds
2.5X to 4X for 5 seconds
2X to 3X for 10 seconds
1.4X to 2X for 30 seconds
1.2X to 1.6X for 60 seconds
1.1X to 1.5X for 120 seconds
1.0X to 1.2X for 300 seconds and stays at this rating for any longer time.

The range of values here is because of manufacturing tolerances. For example, one breaker might trip at 5X at 1 second, the next one on the assembly line might trip at 7X, the next one at 6X, and so on. If they were made to be precise, they'd be 10 times the price.

A #14 copper wire can easily withstand the overcurrent in the above table without dangerous heating, notice how the time increases with current decrease. Heat is a matter of current vs. time.

The starting current of a typical capacitor-start motor is about 6X its running current. For example, say we had a 3/4HP motor, full-load current of 10 amps. The starting (locked-rotor) current would be about 60 amps (4X the rating of the breaker). If power was applied to the motor, and the shaft was jammed, we could expect the breaker to trip somewhere between about 2-1/2 to about 5 seconds.

Rob
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Old 10-29-2008, 03:48 PM   #14
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Tripping Point?


Quote:
Originally Posted by chris75 View Post
You can load a breaker 100%, and i doubt there is anything in residential that would qualify for a continuous load.
Lets see I can name 3 in residential right off the top of my head that can be considered continious loads baseboard heat, water heater (electric not gas), and an oven.
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Old 10-29-2008, 04:02 PM   #15
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Tripping Point?


Quote:
Originally Posted by Pudge565 View Post
Lets see I can name 3 in residential right off the top of my head that can be considered continuous loads baseboard heat, water heater (electric not gas), and an oven.
I give you only fixed electric heat and a water heater... that is it. And I left them out because I figured they were obvious enough.

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